The control of plasmid DNA replication is probably exercised at one or all of the stages in initiation of DNA synthesis. We propose to investigate such control by a thorough exploration of the nature of a temperature-conditional mutation (designated copl) in a plasmid, which results in over-replication of the plasmid at the non-permissive temperature. The lesion has been mapped physically by deletion analysis. We propose to sequence the DNA of both the mutant and wild-type plasmids, in the region where copl maps to discern the base change(s) involved. Further experiments are suggested to confirm the copl location and the nature of cop function in control of replication.